1. Field of the Invention
Human leucocyte antigens (HLA) play a pivotal role in cellular immune responses. They present peptides derived from various proteins to T lymphocytes: HLA class I molecules present peptides derived from intracellular, cytoplasmic, antigens to (CD8 positive) HLA class I restricted cytotoxic T cells [1], whereas HLA class II molecules present peptides derived from exogenous antigens to (CD4 positive) regulatory T lymphocytes [2].
2. Description of the Related Art
The precise definition of such peptide epitopes is of interest for understanding variation in immune responses between individuals, for identifying the pathogenic mechanisms of autoimmune disorders and for designing subunit vaccines. Recently, by eluting peptides from purified class I molecules, Rammensee and colleagues [3] and others [4] showed that particular HLA class I types bind short peptides of 8-10 amino acids in length. Furthermore, they found conserved amino acids at certain positions in the peptides bound to particular class I molecules which are characteristic of the individual HLA, and thereby defined "motifs" for the type of peptide that binds to a given HLA type. This suggests a means of identifying epitopes within previously known protein antigens by selecting short peptide sequences from that antigen that fit the peptide "motif" for a given HLA. However, it is likely that only a small proportion of peptides fitting such a motif are actually epitopes [5].
We have recently developed a novel assay [6], termed an HLA assembly assay, which provides a means of testing whether particular peptides will bind at high affinity to a particular HLA class I molecule, such as HLA-A2. Peptides which act as epitopes would be expected to bind to their particular class I molecule with high affinity, and we found that known HLA-A2 epitopes did so [6]. This suggested to us that combining the techniques of Rammensee et al. to identify conserved residue(s) in peptides binding a particular HLA class I with the HLA assembly assay to screen such peptides for high-affinity binding, would provide a rapid and novel approach to identifying peptides presented by HLA class I molecules that should include most or all HLA class I restricted T cell epitopes in that antigen. Then cytotoxic T cell assays may be undertaken using lymphocytes from individuals exposed to the micro-organism to identify which of the binding peptides are recognised as cytoxic T cell epitopes.